Vitreous coated magnetic material



MarchIO, 1959 A. H. IVERSEN VITREOUS COATED MAGNETIC MATERIAL Filed May19. 1954 United States Patent 2,877,144 7 VITREOUS COATED MAGNETICMATERIAL Arthur H. Iversen, Santa Monica, Calif., assignor to HughesAircraft Company, Culver City, Calif., a corporation of DelawareApplication May 19, 1954, Serial No. 430,841

4 Claims. (Cl. 117-215) This invention relates to hermetically-sealedferromagnetic dielectric materials and a method for sealing the same,and more particularly to vitreous-coated ferromagnetic dielectriccompacts and a method for applying a vitreous coat to such materials.

Ferromagnetic dielectric materials, known in the art simply as ferrites,are crystalline ceramics in which the major constituent is iron oxide.These ceramics are generally formed into a compact by aclosel'yacontrolled sin-' tering process from a powder mixture whichincludes a plurality of bivalent-metal oxides. Ferrite compacts have awide range of physical and electrical properties which are dependent ontheir composition and the sintering technique employed in theirformation.

The ferromagnetic dielectrics are particularly useful in microwaveapplications because at these frequencies they exhibit a characteristicknown as the Faraday effect. However, in use, the heating efiect, powerfactor, and

polarization rotating ability of the ferrite compacts are adverselyaffected by increasing moisture content.

An example of such utilizations of the ferrite compacts,

is shown in copendin'g application, Serial No. 419,259, by Willard A.Hughes, filed onMa'rch 29, 1954. In'this example the ferrite compact isutilized'to rotate incident wave energy 45 degrees for transmission to aload. The reflected energy is rotated another 45,degrees in againtraversing the ferrite compact so that it has vafinal polarization atthe source end of the ferrite; which is normal to that of the incidentenergy. This relationship between the'planes'of polarization of theincidentand reflected energyma kes it: feasible to divert thereflectedenergy out of th e workingsystem and soprevent it from traveling back tothe source.

In addition to the utilization of the'ferrites in microwave-transmissionsystems, they arefalso useful in certain types of electron dischargedevices such as, for example, the traveling wave type; Thesusceptibility of the ferrites to release occluded gases over a longperiod of time is an impediment to: this type of use because of thenecessary high vacuum which'must be maintained in the electron dischargedevice. It follows that thisproblem can be obviated by hermeticallysealingthe ferrite compact inaccorda'nce with the present invention.Accordingly, it follows that if the ferrite compacts can be hermeticallysealed in an envelope or skinin a dry condition, their usefulness will.be very much enhanced. The manner in which thev ferrites are utilized,as well as their inherent physical properties, interposes a number ofdifiiculties in the way of achieving a's'atisfactory hermetic envelope.

lized in high intensity microwave applications which cause considerableheating of the compact. It follows that a sealing skin for the compactmust be capableyof withstanding at temperature of at leastseveralhundred degress centigrade ,without deterioration, cracking orotherwiserdeveloping porosity.- The substances capable of Theferromagnetic dielectric compact or ferrite is im-f "ice performing asealing function at these temperatures are the vitreous products,notably, glass.

An obstacle in the way of utilizing a vitreousv skin onferrite compactsis the inherent susceptibility of the compact to deterioration, i. e.,cracking, due to thermal shock. That is, the tensile and compressivestrengths of theferrite compact are insuflicient to prevent disrup-,tion of the compact due to sudden heating to a high temperature of theouter surfaces thereof. Consequently, since a glass surface must beapplied to the compact at the molten temperature of the glass, it isnecessary to utilize a glass which has a temperature of melting which isnot so high as to cause breakup or cracking of the compact. On the otherhand, because, as previously, mentioned, the ferrite compact can becomequite hot in' operation, it is necessary to utilize a glass which has ahigh enough melting temperature to withstand deterioration of the glassskin or envelope.

In addition to the above problems involved'tin providing a ferritecompact with a vitreous coat or skin, itis also necessary that thecompact with its vitreous skin withstand variations in temperature overseveral hundred degrees centigrade without deterioration of either thecompact or seal. This requires that the vitreous substance uitlized forsealing the compact have a coefficient of thermal expansion suflicientlysimilar to that of the ferrite compact to limit the stresses in eitherthe skin or the, compact itself so as to avoid deterioration of. eitherthe skin or the compact.

"It. follows that a prime objective of this invention vis to provide asealing coat or skin onto a ferromagnetic dielectric compact.

A further objective of this invention is to providev a method forapplying a vitreous coating to ferromagnetic dielectric compacts.

The novel features which are believed to be characteristic of theinvention, both as to its organization and method ofoperation, togetherwith further objects and advantages thereof will become apparent fromthe fol lowing description considered in connection with theaccompanying drawing, made a part of this specification.

In the drawing; Fig. 1" is a partial broken section of a hermeticallysealed ferromagnetic ferrite device of the present inven tion; and aFig. 2 is an enlarged cross-sectional view, section 22.' of the deviceshown .in Fig. 1. p 7

Referring to the figures of the drawing, a typical embodiment of thepresent invention is shown comprising an elongated ferrite compact orslug 10 having a. glass coating or skin 11 which hermetically seals thecompact 10.,from the surrounding medium. shown, the diameter of themiddle portion of thecompact is of the order of from 0.1 to 1.0 inch andits length is generally within the range of from one to four inches, thedimensions being determined by the particular application. Inasmuch asferrites may be used in numerous other forms such as, for example,hollow or solid cylinder, it is readily apparent that the scope of thepresent: invention is not limited to the sizes mentioned or particu--larfconfiguration shown. .7

In order to provide a suitable glass skin on the ferrite compacts, it isfirst necessary to determine the physical properties of the ferrite.Ferrites of many varied and diverse. compositions are commerciallyavailable. Many different compositions are employed because of theunusually large number of combinations of different useful physicalproperties exhibited by all of them. 'Representa tive' samples of aferromagnetic ferrite may contain the following constituents: MnO, MnOCuCO ZnO, Z

Patented Mar. 10, 19,59

In the embodiment mally chemically reactive with lead peroxides andborates but they are not reactive with oxides of aluminum or magnesiumor the carbonatesof sodium or calcium.

, The mineral magnetite, FeOFe O is the well-known, ferro-ferriteoccurring in nature. Past experiments have led to success in makingsynthetic ferrites with both soft" and permanent magnetic properties.Due to the absence of metal components and in view of their densestructure, ferrites have relatively high volume resistivity and highpermeability-in comparison to powdered iron materials. Their specificgravity lies between4 and 5 and their dielectric constant is about 9.These and other peculiar properties have led to a wide field ofapplications, many of which, as previously mentioned, require ferritesto be hermetically sealed.

The thermal coefficient of expansion of ferrites is primarily limited tothe range 71 to 93 inch per inch per C. They have a relatively highmodulus of elasticity and their ultimate strength is usually about10,000 p. s. i., i. e., pounds per square inch, in compression and 2000p. s. i. in tension. These three physical properties are generally themost important when correlated with those of glass, the thermalexpansion coefficient normallybeing the most important of the three.

To produce a hermetic seal that will Withstand high vacuum and largetemperature variations, glass probably has better physical and chemicalproperties than any other material. In obtaining a glass seal for amaterial it is necessary for the glass to wet and to match the material.Since most all glasses wet metal oxides, there is obviously no problempresented with respect to this requirement in coating ferrites sincethey. are almost totally constituted of the oxides of bivalent metals.What is normally meant by matching a glass to a material is to seek outa glass having a thermal coefiicient of expansion near enough to that ofthe material to prevent a structural failure in either by stresses setup by differential expansion. I v

In the manufacture of a good seal, being able to match a glass and amaterial from the standpoint of thermal expansion, i. e., being able tokeep the strains in the glass or the material below their breakingstrength in case their thermal expansions do not match, is notsufiicient' when accomplished only at room temperature because the glassand material must first of all be heated to a high temperature in orderto make the seal. For this purpose, the glass is heated to its workingtemperature, the temperature above that at which it begins to soften orchange its shape. In fact, the glass should be rendered plastic orfluid-like so that it will wet the material and stick thereto. Such atemperature is in the neighborhood of 800 C. or higher,

although, as it will be seen, one type of useful glass hasv a workingtemperature of 560 C.

The working temperature of a glass is to be distinguished from thesoftening temperature which may be as low as 440 C. and which is definedas being the temperature at which the glass becomes suificientlyyielding "so that strains are relieved at an extremely rapid rate orwithin a. very short period, for example, less than one minute. The sealis then allowed to cool directly to room temperature or, as is the moreusual case, the cooling process is temporarily arrested at apredetermined elevated temperature, termed the annealing" point or thelowest temperature below the softening temperature at which 90% of theinternal stresses of the glass will be removed in about fifteen minutes.

In choosing a suitable glass in which to seal a ferrite,"

glass formed in such a batch requires, for melting, temperatures ofabout 1700 C. which temperatures are too expensive and impractical toachieve even with the best commercial glass-furnace refractories. Also,the resultant glass is so viscous, even at melting temperatures, that itcan be homogenized and shaped only with great difiiculty. Consequently,for fiuxing or lowering the melting temperature, some sodium, orpotassium oxide, or both,

usually 319% total, and some correlated amounts of one or more of thestabilizing and modifying oxides such as type glasses, namely,borosilicate glass; and 96% silica.

glass, and pure silica glass or quartz. This listing is more or less inorder of increasing silica content, increasing mechanical durability,improving electrical properties, increasing cost, increasingshapingdifficulty, and decreasing thermal expansion coefiicient.

The composition of these types, given in Table I, are

the analyses of the finished glasses for elements, except oxygen,converted to oxide equivalent.

Table I.-C0mp0siti0ns of commercial glasses Composition, percentComponent Soda-lime Lead Borosili- 96% Silica eate Silica glass S10:7075 (72) Na7O 12-18 (15) K20-.. 1 02.0 5-14 (9) PhD ros A1102" 0.5-2.5(1;

The figures in parentheses give the approximate composition of a typicalmember.

In order to make a successful seal, the expansion of ferrite 10 andglass 11 must be substantially the same over the temperature rangewithin which the glass is elastic. A large difference in expansionbetween them produces stresses which may cause either or both to crackwhen cooled to room temperature. Some degree of differential expansion,however, is tolerable and sometimes desirable.- The relaxationcharacteristics of the glass determine the upper temperature limit towhich matched expansion is essential. Obviously, at temperatures wherethe glass is substantially plastic, equal expansion is no longernecessary.

The glasses which are most useful in practicing the present inventionare then the soft glasses, the sodalime glasses and the lead glasses.The thermal coeflicients C. Although soft glasses may go lower topossibly 10"' inch per inch per C., few will go as high I as 140X10-inch per inch per C. Fused silica or silica glass has a coetficient ofexpansion which is too low for the present purposes, being about5.5X10'- inch per inch per C. and 96% silica glass likewise has toolow,a coefficient of expansion, being about 8 lO-' inch per inch per C.The glasses called borosilicate glasses contain 5% or more of boricoxide. They are also usually too hard to seal a ferrite, the range oftheir thermal expansion coefficients being about 13 to 60x10- inch perinch per C.

Glass in general has an ultimate strength in compression and tensionranging respectively from 90,000 to. 180,000 and from 4,000 to 1,000,000p. s. i. The ultitriate strength in tensioni's normally r'fiuch' 'lessthan 1,000,000 p.;-s. i. except: for very small glassfibers. It. ishowever, pertinent to note tlzat the use ofany glass with a.ferritedemands that the thermal expansion coefficients be well. matched toprevent a structural failure particularlyyin'the ferrite because ofthelow ultimate strength of the latter.

1 Nextin importance to the thermal expansion coelficient' is the modulusof elasticity of a glass. This :is. true be cause the glass should alsobe pliable at low temperatures to be. able to take a large deformationwith very little stress; i. e., its modulus of elasticity should be aslow as possible. Youngs modulus, does not appear to. bees: peciallyrelated to the hardness of a glass or to any particular element ofitscomposition. It is also true that there appears to'be -nocorrelati0nbetween Youngs modulus and thethermal expansion coeflicient. Thismoduljus of elasticity, however, does not normally vvaryappreciably forsoft glasses of commoncompositions andis, therefore, not nearly soimportant as the thermal expansion of-;glass;.-- For example, all vYouugsrrn'cpduli of glasses fall within the range 65. to 127x10 p .s. i. Somego as low as 76x10 p. 's. if-with a thermal expansioncoeflicient. of79.6 inch per inch C. The moduli *ofzhard glasses are. scattered overvan equally wide range,.going as low as 68x10? p. s. i. for a glasshaving a thermal expansion, coefiicient of 32X 10" inch. per inch per..C; and going as high'as 127 10" p. s. i.-f0r thermal expansioncoefi'icient of 42x10- inch. per inch per C. I

pAlthough the soft glasses are generallysatisfactoryfor coating aferrite, there are a few. glasses whichmay be used toaparticularadvantage to reduce the riskz'of structural. failures during. themanufacture of. the seal. and especially: during the handlingof thecomponent materials. For example, in the case of ferrites having athermal expansion coeflicient falling within the range 71 86 10' inchper inch per C., it has been an effective expansivity between 65 and l00l0 inch per inch per C. A glass having characteristics approximatingthese is commonly known as lime glass. This glass is composed of 73.3%SiO 15.6% Na O; 5.4% CaO; 3.8% MgO; 1.4% R 0 0.5% K 0 and has aneffective expansivity of 92i2 10- inch per inch per C., and a Youngsmodulus of 98 l0-' p. s. i.

It has also been found that a seal between the soft glasses known to thetrade as 0080 and 7570 manufactured by the Coming Glass Company may beused in hermetically sealing the ferrites of the above description. TheCorning Glass Company designates clear sealing bulb glass and solderglass with the number 0080 and 7570, respectively. These glasses arekept as closely as possible to a standard chemical composition at alltimes.

In order to apply the glass coating 11 to the ferrite slug 10, asuitable glass of the above-described types is first powdered and thensuspended in a liquid such as, for example, water, methyl alcohol, or asolution of nitrocellulose in amyl acetate. This suspension of powderedglass is then brushed or sprayed uniformly over the ferrite slug 10 oralternatively, the ferrite slug 10 may be immersed in a suspension ofpowdered glass in the solution of nitrocellulose in amyl acetate.

The ferrite slug 10 covered with the suspension of powdered glass isthen heated in a controlled atmosphere to the working temperature of theglass and then slowly cooled. This process is repeated until the glasscoating 11 has a glaze finish and is of a suitable thickness. Thethickness of glass coating 11 depending on the uses to which the ferriteis to be put is of the order of from 0.003 inch to more than 0.06 inch,the thinner coatings being used where permissible in that the electricallosses are less.

Referring again to Fig. 1, it is seen that glass coating 11 of thedevice has elongated pointed ends 12 where this, it is necessary for theelectromagnetic wave to propagate through the medium of the device. Inthat the dielectric constant of ferrite is quite high, there isgenerally an impedance mismatch between the medium surrounding thedevice and the medium of the ferrite slug-10. Thus,

if the glass has a dielectric constant approximately equal to thegeometric mean of the dielectric constants of :the surrounding andferrite mediums, it may be employed to substantially improvetheimpedance match of the ferrite slug 10 to the surrounding medium. Thethicknessof glass points 12 for optimum matching as. measured along thelongitudinal axis of. the device is of the orde.

of one-half guide wavelength.

-A glass suitable for making the glass points 12 isknoivn some ferritesthat they .are sufiiciently porous so as to continuously absorb theglass when heated to the fluid state. This effect is to be avoided inthat the characteristicsofthe ferrite aredeleteriously effected.According toathepresent invention, ferrites of this type are sealed. byfirst applying a coatingof glass having a-high work-.

ing temperature to the ferrite. This coating will generally noteffectively seal the ferrite but it will prevent subsequent coatings ofglass having a lower working temperature from being absorbed into theferrite.

More particularly, this process of hermetically sealing a porous ferriteis as follows:

(1) Suspend powdered Corning glass 0080 in water.

(2) Brush glass 0080 onto the ferrite.

(3) Heat glass 0080 in an oven in air to its sintering temperature, i.e., the temperature at which substantial fusion but not fluidity isindicated, e. g., 700800 C.

(4) Allow the glass and the ferrite to cool.

(5) Brush a liquid suspension of powdered Corning glass 7570 over thesintered 0080 glass.

(6) Heat the ferrite and glasses to the working temperature of the 7570glass, i. e., about 560 C.

(7) Allow both glasses and ferrite to cool.

Corning glass 7570 by itself has utility in the manufacture of theferrite seal of the present invention in that its working temperature is560 C. whereas most other glasses have a working temperature above 800C. The use of this glass reduces the risk of structural failure due tothe low thermal shock resistance of ferrites. Corning glasses 7570 and0080 have thermal expansion coefficients of 84 and 92X 10- inch per inchper C., respectively, and in this regard are useful in preventingdifferential stresses. Youngs modulus of Corning 7570 is within theusual soft glass range. Likewise, Corning 0080 has a Youngs modulus of98x10" p. s. i.

A few other glasses may also be used to hermetically seal a ferritematerial. Among these glasses are clear sealing glass which isdesignated by the present Corning number code by 8870. Its applicationis somewhat limited particularly because of its high dielectricconstant, viz., 9.5; however, its thermal expansion coefiicient is 91 X10- inch per inch per C. and a large advantage accompanying itsemployment is its modulus of elasticity, 76 10-" p. s. i., which iscomparatively low.

What is claimed is:

1. A device comprising an element composed of a porous ferromagneticdielectric ceramic, said element having predetermined physicalcharacteristics; a sintered layer of a first type of glass covering theexposed surface of said element, said first type of glass having apredetermined working temperature; and a coating of a second type ofglass disposed on top of said sintered layer, the working temperature ofsaid second type of glass being more than 80 C. lower than saidpredetermined working temperature, and said first and second types ofglasses having thermal coefficients of expansion of from 65 to 120 X10inch per inch per degree centigrade, whereby said first and second typesof glass provide a seal having physical characteristics substantiallyequivalent to said predetermined physical characteristics of the ferriteelement.

2. The method of hermetically sealing a normally porous ferromagneticdielectric compact employing first and second types of glasses havingfirst and second predetermined softening temperatures, respectively,said first softening temperature being higher than said second softeningtemperature, said method including the steps of producing first andsecond liquid suspensions of said first and second types of glasses,respectively; applying said first liquid suspension to the surface ofsaid compact; heating said compact to the fusing temperature of saidfirst type of glass to produce a coating of said first type of glass onsaid compact that partially seals the surface thereof; applying saidsecond liquid suspension to the coated surface of said compact; andheating said compact to a temperature not less than the softeningtemperature of said second type of glass and less than the softeningtemperature of said first type'of glass.

3. A device comprising an element composed of a porous ferromagneticdielectric ceramic, said element having predetermined physicalcharacteristics; a sintered layer of a first type of glass disposed onthe exposed surface of said element, said first type of glass havingapredetermined working temperature; anda coating of a second type ofglass disposed on top of said sintered layer, the working temperature ofsaid second type of 7 glass being less than-said predetermined workingtemperature, whereby said first and second types ofglass provide a sealhaving physical characteristics substantially equiva lent to saidpredetermined physical characteristics of the ferrite element. r

4. The method of hermetically sealing a normally porous ferromagneticdielectric compact employing first and second types of glasseshaving'first and second predetermined softening temperatures,respectively, said first softening temperature being higher than saidsecond softening temperature, said method comprising the steps ofproducing a glaze of said first type of said glasses over the exposedsurface of said compact, and producing a glaze of said second type ofsaid glasses over the exposed surface of said first type of saidglasses.

References Cited in the file of this patent UNITED STATES PATENTS612,839 Gallinowsky Oct. 25, 1898 1,221,561 Meyer Apr. 3, 1917 1,663,660Hottinger Mar. 27,1928 2,076,869 Tanner Apr. 13, 1937 2,568,881Albers-Schoenberg Sept. 25, 1951 2,570,299 Zademach et al. Oct. 9, 19512,598,371 Gusdorf May 27, 1952 2,643,336 Valensi June 23, 1953 2,677,055Allen Apr. 27, 1954 2,685,539 Woodburn et a1 Aug. 3, 1954 2,745,069Hewitt May 8, 1956 2,748,353 Hogan May 29, 1956 UNITED STATES PATENTOFFICE CERTIFICATE OF CORRECTION Patent No. 2,877,144 March 10, 1959Arthur H. Iversen It is hereby certified that error appears in theprinted specification of the above numbered patent requiring correctionand that the said Letters Patent should read as corrected below.

Column 2, line '70, for "MnO," read MgO, column 4, line 39, Table I,third column, for "0.2" read 0-2 line 62, for "65 X 10 read 6'7 X 10-Signed and sealed this 30th day of June 1959.

Attest:

KARL Hn AXLINE Attesting Oflicer ROBERT cl WATSON Commissioner ofPatents UNITED STATES I PATENT OFFICE CERTIFICATE OF CORRECTION PatentNo. 2,877,l44 March 10, 1959 Arthur H. Iversen It is hereby certifiedthat error appears in the printed specification of the above numberedpatent requiring correction and that the said Letters Patent should readas corrected below.

Column 2, line '70, for "MnO," read MgO, column 4, line 39, 7 Table I,third column, for "0.2" read 0-2 line 62, for "65 X 10 read 6'7 X 10-Signed and sealed this 30th day of June 1959.

Attest:

KARL H. AXLINE Attesting Oflicer ROBERT c. WATSON Commissioner ofPatents

1. A DEVICE COMPRISING AN ELEMENT COMPOSED OF A POROUS FERROMAGNETICDIELECTRIC CERAMIC, SID ELEMENT HAVING PREDETERMINED PHYSICALCHARACTERISTICS; A SINTERED LAYER OF A FIRST TYPE OF GALSS COVERING TRHEEXPOSED SURFACE OF SAID ELEMENT, SAID FIRST RYPE OF GLASS HAVING APREDETERMINED WORKING TEMPERATURE; AND ACOATING OF A SECOND TYPE OFGLASS DISPOSED ON TOP OF SAID SINTERED LAYER, THE WORKING TEMPERATURE OFSAID SECOND TYPE OF GLASS BEING MORE THAN 80*C. LOWER THAN SAIDPREDETERMINED WORKING TEMPERATURE, AND SAID FIRST AND SECOND TYPES OFGLASSES HAVING THERMAL COEFFICIENTS OF EXPANSION OF FROM 65 TO 120X10-7INCH PER INCH PER DEGREE CENTIGRADE, WHEREBY SAID FIRST AND SECOND TYPESOF GLASS PROVIDE A SEAL HAVING PHYSICAL CHARACTERISTICS SUBSTANTIALLYEQUIVALENT TO SAID PREDETERMINED PHYSICAL CHARACTERISTICS OF THE FERRITEELEMENT.